The present invention relates to a thermal flow sensor implementing at least one nanoelectromechanical resonator (or NEMS for “nanoelectromechanical system”) or microelectromechanical resonator (or MEMS for “microelectromechanical system”), to determine the concentration of the components of a gas from its thermal characteristics, and to a gas sensor comprising at least one such sensor.
A thermal flow sensor refers to any sensor measuring a heat exchange between the body of the sensor and the fluid medium in which the sensor is positioned. This thermal flow sensor is for example a gas sensor or a pressure sensor.
A system capable of determining the analyte composition of a gas may be used to detect and quantify the analytes at the outlet of a chromatography column, more particularly a chromatography micro-column, the latter making it possible to temporarily separate the different gaseous elements of a complex mixture. The sensor serves to quantify the relative concentration of the analytes of the gas to be analyzed successively arriving on its surface. The analytes are mixed in a gas, called carrier gas, that is sent into a chromatography column and on the sensor at a fixed speed.
The carrier gas may be dry air or an inert gas, for example.
Several types of sensor exist that may be positioned at the outlet of the chromatography column.
Flame ionization detector (FID) sensors.
The gases to be analyzed are burned under a hydrogen flow creating ions and electrons. The charged particles are collected by electrodes and the generated current is measured using a picoammeter. On the one hand, the sensor only allows the detection of organic components. On the other hand, it requires a flow of hydrogen, and the produced quantity of ions remains low. Lastly, the size of the sensor cannot be reduced.
Optical sensors also exist, the operating principle of which is generally based on infrared absorption of an optical flow. The sensors are suitable for detecting carbonaceous elements. However, to be able to detect other types of gases, the number of laser sources would need to be multiplied, which would considerably increase the complexity and cost of such an apparatus. The sensors are also difficult to miniaturize.
Electronic sensors, the detection principle of which is based on varying an electrical property (electrical resistance, resistance, surface potential) induced by the presence of gas molecules on its surface. These sensors require surface functionalization. The macroscopic sensors are relatively insensitive. Micrometric or nanometric sensors suffer from drift problems, i.e., random long-term drifts in the property to be measured and extreme sensitivity to the initial surface states. They must also be functionalized.
Thermal conductivity detector (TCD) sensors also exist. A TCD may comprise a wire brought to a high temperature, the electrical resistance of which is measured. The wire has a given temperature for a given gas. When the gas changes, the properties of the thermal environment (thermal conductance, viscosity, heat convection) change, which causes a variation in the temperature of the wire. This variation in turn causes a change of electrical resistance that is detected through a measurement bridge. The higher the temperature of the TCD sensor, the better its resolution. It is therefore necessary to work in an oxygen-free environment to prevent the wire from burning. Generally, the TCD wire must be placed under a helium carrier gas flow or hydrogen carrier gas flow. Using a rare gas makes it possible to have a considerable contrast relative to the air. This represents a significant limitation of the detector. Furthermore, a major contrast of thermal constants exists between these light gases and the analytes to be detected, which makes the system more sensitive than under a simple flow of dry air.
Gravimetric sensors also exist. They involve measuring the mass quantity of the target gas adsorbed at the surface of the sensor.
Generally, the sensor is a system vibrating at a unique oscillation frequency. The oscillation frequency shift of the system is measured, which results from the addition of mass, also called gravimetric effect, toward the low frequencies caused by the adsorption of the gas. The surface of the sensor is functionalized to sense the gas. These sensors have a high sensitivity for large gaseous molecules, but a lower concentration measurement sensitivity for very light and/or volatile molecules. This type of sensor is described in the document Fanget, S. Hentz, P. Puget, J. Arcamone, M. Matheron, E. Colinet, P. Andreucci, L. Duraffourg, E. Myers, M. L. Roukes, “Gas sensors based on gravimetric detection—A review”, Sensors and Actuators, B: Chemical, 160(1), 2011, 804-821.